High-voltage fuse with one piece fuse links

ABSTRACT

A high-voltage fuse for interrupting both protracted overload currents and major fault currents and operating as a currentlimiting fuse when interrupting major fault currents. The fuse includes one or more fusible elements each operating in the fashion of a composite fusible element including serially connected ribbon sections and a wire section interposed between, and spot-welded to, the ribbon sections. In fuses according to this invention one single stamping, i.e. one single length of sheet silver stock takes the plate of a spot-welded composite of ribbon sections and a wire section. The wire like effect is achieved by providing an oblong hole, or window, in the center region of the ribbon fuse link, resulting in the formation of two wire-like strips of small cross-sectional area to both sides of the oblong hole or window.

United States Patent Kozacka et al. 1 June 20, 1972 s41 HIGH-VOLTAGEFUSE WITH ONE FOREIGN PATENTS OR APPLICATIONS PIECE FUSE LINKS 864,9435/1941 France ..337/229 [72] Inventors: Frederick J. Kozaeka, SouthHampton;

Richard Belch", Hampton Falls, both Primary Examiner-Bernard A. Gilheanyof Assistant Examiner-Dewitt M. Morgan Attorney-Erwin Salzer [73]Assignee: The Chase-Shawmut Company, Newbuyp MaSS- [57] ABSTRACT [22]Filed: Feb. 17, 1971 A high-voltage fuse for interrupting bothprotracted overload currents and major fault currents and operating as acurrent- [2]] Appl' N l16o37 limiting fuse when interrupting major faultcurrents. The fuse includes one or more fusible elements each operatingin the 52 us. (:1 ..337/160, 337/292, 337/295 fashion of a compositefusible element including serially 5 Int c mflmh 35/10, 011 5 12 nectedribbon sections and a wire section interposed between, 581 Field ofSearch ..337/159, 160, 161, 162, 166, and spot-welded the ribbonsections In fuses according I9 337/229, 292 293, 295 296 this inventionone single stamping, i.e. one single length of sheet silver stock takesthe plate of a spot-welded composite of ribbon sections and a wiresection. The wire like effect is [56] Reemnces Cited achieved byproviding an oblong hole, or window, in the UNITED STATES PATENTS centerregion of the ribbon fuse link, resulting in the formation of twowire-like strips of small cross-sectional area to both 3,386,062 5/1968Kozacka 8! al. ..337/160 Sides of the oblong hole or window 3,471,81810/1969 Koch ...337/295 X 3,571,775 3/1971 Kozacka et a] ..337[295 X 12Claims, 7 Drawing Figures PATENYEDJUHZO I872 3,671,909

FIG.|

INVENTORS:

FREDERICK J. KOZACKA RICHARD A. BELCHER BY WWW WW ATTY.

HIGH-VOLTAGE FUSE WITH ONE PIECE FUSE LINKS BACKGROUND OF THE INVENTIONIt is an old onerous problem in the fuse art to provide fuses which arecapable of effectively interrupting both major fault currents, or shortcircuit currents, as well as overload currents of inadmissible duration.US Pat. No, 3,386,062 to Frederick J. Kozacka et al., May 28, 1969 forELECTRIC CARTRIDGE FUSE FOR INTERRUPTING PROTRACT ED OVERLOAD CURRENTSAND MAJOR FAULT CURRENTS discloses a fuse that performs particularlywell the aforementioned two functions. This is primarily due to theparticular geometry of its ribbon fuse link of silver. The fuse linkincludes a plurality of serially related necks, or points of reducedcross-sectional area, which are relatively short, and one neck, or pointof reduced cross-sectional area, which is relatively long, and arrangedin series with the aforementioned plurality of relatively short necks.It is desirable to adapt the aforementioned geometry to fuses having arelatively high voltage rating, e.g. a voltage rating from about 2.5 kVto kV. This calls for a substantial increase of the total length, and ofthe length of the relatively long neck, of the fuse link structure ofU.S. Pat. No. 3,386,062. The necessary increase in the length of theneck renders the fuse link structure extremely fragile, and makes itvery difficult to handle the same. It can only be handled byparticularly skilled labor, and even if handled by skilled labor withreasonable care the rate of damage to such fuse links is relativelyhigh. This is particularly troublesome since the damage to such a fuselink cannot normally be detected by visual inspection thereof, but canonly be discovered by conducting electrical tests after the fuse hasbeen assembled. The fact that quality control can only discover slightlydamaged yet unacceptable fuse links upon assembly of a fuse greatlyincreases the cost of production, particularly since the cost ofproduction of each'high-voltage fuse unit are relatively high.

Because of the difficulties involved in forming and in processing ribbonfuse links having long neck portions intended for the interruption ofprotracted overload currents, a type of high-voltage fuses has beendeveloped wherein the long neck portions of the fuse link are formed bysilver wires. The wires interconnect conductively ribbon sections of thefuse link which form a plurality of serially related relatively shortnecks or points of reduced cross-sectional area. The latter form seriesbreaks upon occurrence of major fault currents. Such high-voltage fusesare disclosed and claimed in the copending patent application ofFrederick J. Kozacka et al., Ser. No. 016,116, filed Mar. 30, 1970 forHIGH-VOLTAGE FUSE HAVING A PLURALITY OF HELICALLY WOUND RIBBON FUSELINKS now US. Pat. No. 3,571,775 issued Mar. 23, 1971 While theperformance of high-voltage fuses of the aforementioned characterincluding fusible elements having wire sections forming relatively longoverload current interrupting necks and having ribbon sections formingrelatively short major fault current interrupting necks is satisfactory,the process of affixing the wire sections to the ribbon sectionsinvolves considerable difficulties. Affixing the wire sections to theribbon section calls for spot-welding operations which are, in essence,manual operations calling for a certain skill and also requiring manualhandling of fragile ribbon fuse links which may result in damage to thelatter. A spot-weld may result in sufficient mechanical strength of thejoint without reducing the contact resistance as much as necessaryand/or desirable. In such instances the spot weld. must be covered by asolder joint to minimize contact resistance, and this means anadditional operation which is not simple by any means, and furtherincreases the cost of production.

The principal object of this invention is to provide high-voltage fuseswhich perform substantially in the same way as the fuses disclosed inthe above referred-to U.S. Pat. No. 3,386,062, and which can bemanufactured at reasonable or lower cost, its fuse link being arelatively rugged one-piece structure rather than a fabricated,spot-welded structure.

SUMMARY OF THE INVENTION High-voltage fuses embodying this inventioninclude a tubular casing of electric insulating material and a pair ofterminal elements arranged at the ends of the casing and closing thelatter. The casing is filled with quartz sand. A ribbonfuse link ofsilver conductively interconnecting the pair of terminal elements issubmersed in the filler of quartz sand. The fuse link has a plurality ofserially related relatively short necks arranged in relatively longaxially outer portions thereof, and the fuse link has a relatively longneck arranged between the axially outer portions thereof. Each of theplurality of short necks is formed by a circular perforation positionedin the center of the fuse link and having a predetermined diameter. Theaforementioned relatively long neck is formed by an oblong perforationin the center of the fuse link. The width of said oblong perforation isequal to the diameter of each of said circular perforations. This oblongperforation results in the formation by the stock of the fuse link of apair of parallel narrow strips of silver conductively interconnectingsaid axially outer portions thereof. An overlay of a silver-severingmetal having a lower fusing point than silver is arranged on said narrowstrips of silver.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is in substance a verticalsection of a fuse embodying this invention taken along [-1 of FIG. 2;

FIG. 2 is a transverse section of the fuse shown in FIG. 1 taken alongIIII of FIG. 1;

FIG. 3 shows in side elevation a modification of the fuse link structureof FIG. 1 intended to be arranged in the general assembly shown in FIG.1, and described in connection therewith;

FIG. 3a is a front elevation of a portion of the fuse link structure ofFIG. 3;

FIG. 4 shows in elevation a portion of another fuse embodying thisinvention;

FIG. 5 shows in side elevation another modification of the fuse linkstructure of FIG. I intended to be arranged in the general assemblyshown in FIG. 1 and described in connection therewith; and

FIG. 5a is a front elevation of a portion of the fuse structure of FIG.5.

' DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,numeral 1 has been applied to indicate a tubular casing of electricinsulating material, preferably a laminate of melamine and glass cloth.A pair of terminal elements is arranged at the ends of casing 1, closingthe latter. The terminal elements are preferably formed by metal plugs2, press-fitted into the ends of casing 1, and maintained in position bysteel pins 3 projecting transversely through casing 1 into plugs 2.Terminal caps or ferrules 4 are mounted on the ends of casing l, andtheir axially inner surfaces are clamped by means of hexagonal screws 5projecting through caps or ferrules 4 into plugs 2 against the axiallyouter end surfaces of plugs 2. Casing 1 contains a body of quartz sand6, the volume of which is substantially equal to the volume of casing 1.Four ribbon fuse links 7 of sheet silver interconnect conductively thepair of terminal elements or terminal plugs 2, and are submersed in thequartz sand filler 6. Each fuse link 7 has a plurality of seriallyrelated relatively short necks arranged in the axially outer portionsthereof. The letter E has been applied to indicate the length of theaxially outer portions of fuse links 7. Each of the relatively shortnecks of fuse link 7 is formed by a circular perforation 7a positionedin the center line of the respective fuse link 7. The neck proper is theportion of fuse link metal situated to both sides of a circularperforation 7a, and the length of each short neck is equal to thediameter of each perforation 7a. Perforations 7a are spacedequidistantly and have equal diameters. The letter X indicates thelength of the axially inner portion of fuse link 7 which is considerablyless than the length L of the axially outer portions thereof. Theaxially inner portion of fuse link 7 is formed by a relatively long neckwhich, in turn, is formed by an oblong perforation or elongated window7b. Oblong perforation or elongated window 7b is laterally bounded by apair of parallel narrow strips 70 of silver conductively interconnectingthe axially outer portions of fuse link 7. An overlay 8 of asilver-severing metal having a lower fusing point than silver isoperatively related to strips 70 to sever the latter upon fusion of theoverlay 8. An overlay 8 is preferably arranged in the center of each ofstrips 70, but may be arranged at other points thereof, e.g. near theends thereof, as long as it effects a break in strips 7c incident to itsfusion. The ends of ribbon fuse links 7 are bent 90 and abut against theaxially inner end surfaces of terminal plugs 2, and are firmly clampedagainst the latter by screws 10 projecting at right angles into theaxially inner end surfaces of terminal plugs 2.

Upon occurrence of relatively small overloads of inadmissible durationthe overlays 8 on silver strips 7c melt and sever the latter, thusinitiating arcing. The bumback length x of strip 70 may be sufficient tocause extinction of low current arcs, but if sufficient low currentsarcs may be further elongated and quenched by burning into the axiallyouter portions of ribbon fuse link 7. It has been found that the largerthe number of parallel current paths, the more effective the operationof the fuse on occurrence of small overload currents. This is one of thereasons why the low overload current interrupting performance of a fuselink such as shown in FIG. 1 is superior to the low current-interruptingperformance of a fuse having a fuse link which has the geometry shown inthe above referredto US. Pat. No. 3,386,062.

Upon occurrence of major fault currents melting is initiated at thepoints of smallest cross-sectional area of the ribbon fuse link 7. Theprocess of manufacturing fuse links 7 is simplified if the diameter ofcircular perforations 7a and the width w of the oblong perforation 7bare equal. This calls for certain precautions in regard to the selectionof this particular parameter. To be more specific, the diameter of thecircular perforations 7a must be at least in the order of 60 percent ofthe width W of ribbon fuse links 7. Particularly good performance inregard to limiting surge voltage incident to inter ruption of majorfault currents was achieved with fuse links having circular holes, orperforations, 7a whose diameter was 60-70 percent of the width W of thefuse links 7, and whose oblong perforation has a width w of 60-70percent of the width W of fuse links 7. Excellent results were obtainedwith fuse links wherein the cross-sectional area of the short necksformed by perforations 7a at the narrowest points of the necks and thecross-sectional area of the long necks formed by perforation 7b wasabout 36 percent of the cross-sectional area of ribbon fuse links 7, thethickness of the latter being uniform throughout the entire lengththereof. A reduction of the diameter of holes 70 and of the width w ofperforation 7b to 53 percent of the width of fuse links 7 resulted inexcessive surge voltages incident to interruption of major faultcurrents.

The criticality of the aforementioned parameters can be readilyexplained. If the cross-section of the short necks and of the long neckis equal, both kinds of necks fuse simultaneously on major faultcurrents, tending to result in excessive surge voltages. This tendencycan be minimized by imparting a relatively large diameter to thecircular perforations 7a, i.e. a diameter of at least 60 percent of thewidth W of fuse links 7. The larger the diameter of perforations 7a inrelation to the width W of link 7, the smaller the ratio of the smallestcrosssectional area of the necks formed by perforations 7a to the lengthof the necks formed by perforations 7a (which length is equal to thediameter of perforations 7a). The smaller the aforementioned ratio, thelarger the rate of change of crosssectional area along each neck, andthe smaller the rate of change of current di/dt incident to bumback of aneck formed by a circular perforation from the point of smallestcross-sectional area of the neck to its point of largest cross-sectionalarea where the neck ends, and where its cross-sectional area becomesequal to the cross-sectional area of fuse links 7 at the points oflargest cross-sectional area thereof.

FIGS. 3,3a and 5 and 5a refer to fuse link structures having axiallyouter portions of relatively hard resilient silver bent to form a pairof loaded, self-supporting extension springs 9',9". The pair of narrowstrips 7c',7c" formed by oblong windows or perforations 7b',7b" arearranged over centers of the axially outer portions of fuse links 7',7"forming extension springs 9',9". The axially inner portions of fuselinks 7',7 having windows 7b,7b" are substantially planar.

In FIGS. 3 and 3a the reference character 7a has been applied toindicate the circular perforations forming relatively short necks andreference character 8' has been applied to indicate overlays of tin, ora like low fusing point silver-severing metal, arranged in the centersof narrow silver strips 7!). The axially outer portions of fuse link 7which form loaded extension springs 9 are bent transversely at alurality of serially related equidistant points so as to assume azig-zag shape. Strips 7c are arranged over the centers of springs 9' andare, therefore, under tension in a direction longitudinally thereof bythe action of loaded springs 9. The axially outer ends of springs 9 maybe screwed against a pair of terminal plugs as shown in FIG. 1, andexplained more in detail in connection with FIG. 1. Each bend in fuselink 7 is arranged at a point where one of the perforations 7a formingrelatively short necks is located, and one or more perforations 7a arearranged between each bend in fuse link 7'.

According to FIG. 4 a ribbon fuse link 7 having the same geometry as thefuse link 7 of FIG. 1 is wound helically around a mandrel 10" ofinsulating material. Link-supporting mandrel 10" may form a spacerbetween terminal plugs of the kind shown in FIG. 1, and may consist of aplurality of angularly displaced insulating plates having radially outeredges engaging grooves provided in the axially inner end surfaces of theterminal plugs of the structure of FIG. 1. A fuse of this character hasbeen disclosed in detail in the cO-pending patent application ofFrederick J. Kozacka for HIGH-VOLTAGE FUSE, filed Nov. 13, 1969, Ser.No. 876,445, now US Pat. No. 3,599,138 issued Aug. 10, 1971. Referencemay be had to this patent for more detailed information in regard to thestructure of FIG. 4 which has only diagrammatically been shown therein.The fuse of FIG. 4 differs from the fuse disclosed in U.S. Pat. No.3,599, l 38 only inasmuch as the former includes a fuse link whosegeometry differs from the geometry of the ribbon fuse link of US. Pat.No. 3,599,138.

Referring now more particularly to FIGS. 5 and 5a, these figures showanother ribbon type of fuse link which may be substituted for the ribbonfuse link of FIG. I. The fuse link of FIGS. 5 and 5a includes twoaxially outer portions of relatively hard silver which are woundhelically to form a pair of loaded extension springs. The axially outerportions of fuse links 7" are provided with circular perforations whichare arranged equidistantly and form necks of relatively short length. Aplanar portion of fuse link 7" is arranged over the centers of helicalspring windings 9", and provided with an oblong perforation 7b" forminga relatively long neck consisting of two parallel strips 70'' of silver.The latter support near the center thereof an overlay 8" of tin, or thelike low fusing point silversevering metal in form of a solidifieddroplet. The length of the neck formed by oblong perforation 7b" is alarge multiple of the length of the necks formed by perforations 7a", ora large multiple of the diameter of perforations 7a".

The above may be illustrated by some numerical examples. The developmentand successful testing of fuses embodying this invention included fuseshaving a voltage rating of 2.5, 5 and 8.3 kV. The width of the ribbonfuse link was 0.086 inch and its thickness was 0.0060.0ll inch,depending upon desired current-carrying capacity. The spacing betweencircular perforations was 0.203 inch and the length of the oblongperforation in the axially inner portion of the ribbon fuse link was 1%inches. The diameter of the circular perforations was 0.055 inch. In thefuse link for the 8.3 kV voltage rating there were 60 circularperforations to each side of the oblong perforation of 1% inches.

We claim as our invention:

1. A high-voltage fuse including a. a tubular casing of electricinsulating material;

b. a pair of terminal elements arranged at the ends of said casing andclosing said casing;

c. a filler of quartz sand inside said casing;

d. a ribbon fuse link of silver conductively interconnecting said pairof terminal elements and submersed in said filler, said fuse link havinga-plurality of serially related relatively short necks arranged inrelatively long axially outer portions thereof, and said fuse linkhaving a relatively long neck arranged between said axially outerportions thereof, each of said plurality of relatively short necks beingformed by a circular perforation positioned in the center of said fuselink and having a predetermined diameter, and said relatively long neckbeing formed by an oblong perforation having a width equal to saidpredetemiined diameter and arranged in the center of said fuse link andresulting in the formation by the stock of which said fuse link is madeof a pair of parallel narrow strips of silver having a spacing equal tosaid predetermined diameter conductively interconnecting said axiallyouter portions of said fuse link; and

e. overlay means of a silver-severing metal having a lower fusing pointthan silver on said narrow strips of silver.

2. A high-voltage fuse as specified in claim 1 wherein saidpredetermined diameter of said circular perforation is at least in theorder of 60% of the width of said fuse link.

3. A high-voltage fuse as specified in claim 1 wherein said axiallyouter portions of said fuse link are of relatively hard resilient silverand are bent to form a pair of loaded extension springs, and whereinsaid pair of narrow strips is arranged over centers of said axiallyouter portions of said fuse link.

4. A high-voltage fuse as specified in claim 1 wherein said axiallyouter portions of said fu'se link are of relatively hard resilientsilver and are helically wound to form a pair of loaded extensionsprings, and wherein said narrow pair of strips is arranged over centersof said axially outer portions.

5. A high-voltage fuse as specified in claim 1 wherein said axiallyouter portions of said fuse link are of relatively hard resilient silverand are bent transversely at a plurality of serially related points toform a pair of zig-zag shaped loaded extension springs, and wherein saidnarrow pair of strips is arranged over centers of said axially outerportions of said fuse link.

6. A high-voltage fuse as specified in claim 1 wherein each of said pairof narrow strips has substantially midway between the ends thereof anoverlay of a silver-severing metal having a lower fusing point thansilver.

7. A ribbon fuse link as specified in claim 6 wherein each of said pairof axially outer portions is bent helically to form a helical extensionspring.

8. A ribbon fuse link for high-voltage fuses including a ribbon ofrelatively hard sheet silver having an axially inner portion ofrelatively short length and a pair of axially outer portions beingrelatively long in comparison to said axially inner portion, each ofsaid pair of axially outer portions being bent to form an extensionspring, and each of said pair of axially outer portions having aplurality of serially related points of reduced cross-sectional area andpredcten'nined length, each of said serially related points of reducedcross-sectional area being formed by a circular perforation having apredetermined diameter, said axially inner portion being substantiallyplanar, arranged over the centers of said pair of axially outer portionsand formed by a pair of parallel strips which are much longer than saidlength of each of said plurality of points of reduced cross-sectionalarea, said pair of parallel strips being separated by an oblong gaphaving a width equal to said predetermined diameter of said circularperforations, said pair of parallel strips being formed by the sameribbon of sheet silver as said pair of axially outer portions of saidribbon fuse link and said pair of parallel strips being provided withoverlay means of a silver-severing metal having a lower fusing pointthan silver.

9. A ribbon fuse link as specified in claim 8 wherein each said circularperforation has a diameter of at least 60 percent of the width of saidribbon of relatively hard silver.

10. A ribbon fuse link as specified in claim 8 wherein said diameter ofeach of said circular perforation is 60 to 70 percent of the width ofsaid ribbon of relatively hard silver.

11. A ribbon fuse link as specified in claim 8 wherein each of said pairof axially outer portions is bent transversely at a plurality ofserially related points to form a zig-zag shaped extension spring.

12. In a high-voltage fuse the combination of a. an elongated mandrel ofelectric insulating material; and

b. a ribbon fuse link of silver wound helically around said mandrel,said ribbon fuse link having an axially inner portion of relativelyshort length and a pair of axially outer portions being relatively longin comparison to said axially inner portion, each of said pair ofaxially outer portions having a plurality of serially related circularperforations having a diameter at least in the order of 60 percent ofthe width of said ribbon fuse link, said axially inner portion having anoblong perforation having a width equal to said diameter of saidcircular perforations, the length of said oblong perforation being alarge multiple of said diameter of said circular perforations, saidoblong perforation being bounded laterally by a pair of parallel narrowstrips each formed by the same ribbon as any other portion of saidribbon fuse link, and each of said pair of parallel narrow strips beingprovided with overlay means of a silver-severing metal having a lowerfusing point than silver.

1. A high-voltage fuse including a. a tubular casing of electricinsulating material; b. a pair of terminal elements arranged at the endsof said casing and closing said casing; c. a filler of quartz sandinside said casing; d. a ribbon fuse link of silver conductivelyinterconnecting said pair of terminal elements and submersed in saidfiller, said fuse link having a plurality of serially related relativelyshort necks arranged in relatively long axially outer portions thereof,and said fuse link having a relatively long neck arranged between saidaxially outer portions thereof, each of said plurality of relativelyshort necks being foRmed by a circular perforation positioned in thecenter of said fuse link and having a predetermined diameter, and saidrelatively long neck being formed by an oblong perforation having awidth equal to said predetermined diameter and arranged in the center ofsaid fuse link and resulting in the formation by the stock of which saidfuse link is made of a pair of parallel narrow strips of silver having aspacing equal to said predetermined diameter conductivelyinterconnecting said axially outer portions of said fuse link; and e.overlay means of a silver-severing metal having a lower fusing pointthan silver on said narrow strips of silver.
 2. A high-voltage fuse asspecified in claim 1 wherein said predetermined diameter of saidcircular perforation is at least in the order of 60 percent of the widthof said fuse link.
 3. A high-voltage fuse as specified in claim 1wherein said axially outer portions of said fuse link are of relativelyhard resilient silver and are bent to form a pair of loaded extensionsprings, and wherein said pair of narrow strips is arranged over centersof said axially outer portions of said fuse link.
 4. A high-voltage fuseas specified in claim 1 wherein said axially outer portions of said fuselink are of relatively hard resilient silver and are helically wound toform a pair of loaded extension springs, and wherein said narrow pair ofstrips is arranged over centers of said axially outer portions.
 5. Ahigh-voltage fuse as specified in claim 1 wherein said axially outerportions of said fuse link are of relatively hard resilient silver andare bent transversely at a plurality of serially related points to forma pair of zig-zag shaped loaded extension springs, and wherein saidnarrow pair of strips is arranged over centers of said axially outerportions of said fuse link.
 6. A high-voltage fuse as specified in claim1 wherein each of said pair of narrow strips has substantially midwaybetween the ends thereof an overlay of a silver-severing metal having alower fusing point than silver.
 7. A ribbon fuse link as specified inclaim 6 wherein each of said pair of axially outer portions is benthelically to form a helical extension spring.
 8. A ribbon fuse link forhigh-voltage fuses including a ribbon of relatively hard sheet silverhaving an axially inner portion of relatively short length and a pair ofaxially outer portions being relatively long in comparison to saidaxially inner portion, each of said pair of axially outer portions beingbent to form an extension spring, and each of said pair of axially outerportions having a plurality of serially related points of reducedcross-sectional area and predetermined length, each of said seriallyrelated points of reduced cross-sectional area being formed by acircular perforation having a predetermined diameter, said axially innerportion being substantially planar, arranged over the centers of saidpair of axially outer portions and formed by a pair of parallel stripswhich are much longer than said length of each of said plurality ofpoints of reduced cross-sectional area, said pair of parallel stripsbeing separated by an oblong gap having a width equal to saidpredetermined diameter of said circular perforations, said pair ofparallel strips being formed by the same ribbon of sheet silver as saidpair of axially outer portions of said ribbon fuse link and said pair ofparallel strips being provided with overlay means of a silver-severingmetal having a lower fusing point than silver.
 9. A ribbon fuse link asspecified in claim 8 wherein each said circular perforation has adiameter of at least 60 percent of the width of said ribbon ofrelatively hard silver.
 10. A ribbon fuse link as specified in claim 8wherein said diameter of each of said circular perforation is 60 to 70percent of the width of said ribbon of relatively hard silver.
 11. Aribbon fuse link as specified in claim 8 wherein each of said pair ofaxially outer Portions is bent transversely at a plurality of seriallyrelated points to form a zig-zag shaped extension spring.
 12. In ahigh-voltage fuse the combination of a. an elongated mandrel of electricinsulating material; and b. a ribbon fuse link of silver wound helicallyaround said mandrel, said ribbon fuse link having an axially innerportion of relatively short length and a pair of axially outer portionsbeing relatively long in comparison to said axially inner portion, eachof said pair of axially outer portions having a plurality of seriallyrelated circular perforations having a diameter at least in the order of60 percent of the width of said ribbon fuse link, said axially innerportion having an oblong perforation having a width equal to saiddiameter of said circular perforations, the length of said oblongperforation being a large multiple of said diameter of said circularperforations, said oblong perforation being bounded laterally by a pairof parallel narrow strips each formed by the same ribbon as any otherportion of said ribbon fuse link, and each of said pair of parallelnarrow strips being provided with overlay means of a silver-severingmetal having a lower fusing point than silver.